This invention relates to coupling irises employed in the coupling of electromagnetic power through a wall from one microwave structure to another microwave structure and, more particularly, to an iris formed of an aperture having at least one tooth extending inwardly from a periphery of the aperture for increasing magnetic polarizability of the aperture resulting in an increased coefficient of coupling of an electromagnetic field between the two microwave structures.
Microwave structures such as waveguides and the cavities of filters are formed of enclosing walls which contain electromagnetic waves and sustain various modes of vibration of the waves. Such structures may be contiguous with each other with the enclosed regions of the contiguous structures being separated by a common wall. In order to couple electromagnetic power between the two contiguous microwave structures, it is common practice to place a coupling iris in the common wall. The iris may be formed as an elongated aperture in the shape of a slot, or a pair of intersecting slots such as a crossed slot, by way of example. Further examples in the shape of the aperture are a square-shaped aperture and a circular aperture. The shapes and sizes of the apertures are selected to provide a desired magnitude of coupling coefficient, this being the ratio between a coupled field component to the incident field component. The shape,size, and location of the aperture also provides for selectively coupling specific modes of vibration of electromagnetic waves.
A microwave structure of particular interest is a cylindrical resonator in a microwave filter. In such a filter, it is useful to obtain higher order circular modes of propagation of electromagnetic waves, particularly TE.sub.121 mode. It is desirable to obtain a sufficiently high coupling of electromagnetic .waves into a high Q TE.sub.121 cylindrical resonator mode for a microwave filter application, the term, Q, being the ratio of energy stored to energy dissipated per cycle.
A problem arises in that available slot shapes of standard coupling irises must be very large to provide the desired high amout of coupling for the TE.sub.121 mode, as well as for other higher order cavity resonator modes. However, the use of the larger apertures introduces a further problem in that the presence of an overly large aperture in a cavity wall disrupts excessively the electromagnetic fields in the cavity and; furthermore, degrades the cavity Q to an unacceptably low value. As a result, the introduction of an enlarged aperture to provide the increased amount of coupling has defeated the utility of the filter in operating with higher order modes at low loss.